Is there enough water for people and nature? Impacts of flow reduction on fish and aquatic invertebrates in small streams

Abstract

Anthropogenic flow reduction from dams and stream diversions can influence the distribution and structure of biotic communities by altering the physical and chemical properties of their habitat. The consequences of water abstraction for the fish and invertebrate communities of small, marginal, headwater streams are not clear. Therefore, I investigated the effects of water abstraction on (1) invertebrate and (2) fish communities in three Central Otago upland catchments.

My first study compared macroinvertebrate populations as well as habitat availability and suitability above and below stream diversions in permanent flowing reaches of five Central Otago streams: Kye Burn (3), Swin Burn (1) and Ida Burn catchments (1). I used traditional species–composition measures and the macroinvertebrate trait–based community approach for detecting changes in communities, and impacts were assessed over a gradient of low (38%) to high (86%) flow reduction. Abstraction rates as low as 38% caused significant negative responses in invertebrate community indices, species and traits. The greatest impacts were observed at sites where physical habitat was reduced the most. The ability of species–composition measures and trait–based community approaches to detect impacts at low levels of flow reduction was variable and the greatest results were achieved when they were used together. The trait responses appeared to be highly correlated with taxa responses, which made confident interpretation of individual trait responses difficult.

Previous analysis of the broad scale patterns in fish distributions in the Manuherikia catchment (Central Otago) suggested that water extraction in small streams may act to mediate the impact of introduced predators on native galaxiids, by exacerbating low flow conditions that are unfavourable for the invasive cool–water salmonids. To test this hypothesis, my second study focuses on the Ida Burn catchment, a small, semi–arid tributary of the Manuherikia River. The Ida Burn is intermittent during the summer due to a combination of low rainfall and heavy abstraction. Invasive trout are present, and notably, a large population of threatened roundhead galaxiids (Galaxias anomalus) in the lower reaches that are usually only found above barriers to trout migration. My study examined information on hydrology, habitat, and fish populations at the finer scales of reach and individual catchment to critically evaluate the factors influencing galaxiid persistence. My results indicate that the presence of predatory trout is a main driver of galaxiid distribution, although trout distribution was strongly driven by the co–occurring process of habitat degradation resulting from reduced flows. Therefore, the hydrological disturbance resulting from water abstraction was protecting the native galaxiids from being eliminated by trout predation in the lower Ida Burn catchment. Water abstraction combined with an extended drought however, completely desiccated large areas of the remaining galaxiid habitat and greatly impacted their overall abundance. Therefore, I recommend implementing artificial barriers and restoring more natural low flows to the catchment.